PENSACOLA , Florida -LRB- CNN -RRB- -- A fourth suspect has been arrested in the shooting deaths of a Gulf Coast couple known for adopting special-needs children , authorities said late Monday .

Police say they have evidence that places Gary Lamont Sumner at the crime scene .

Gary Lamont Sumner faces a murder charge for his alleged role in the deaths of Byrd and Melanie Billings on Thursday , Escambia County , Florida , Sheriff David Morgan said .

Sumner was pulled over in a traffic stop in Okaloosa County Sunday and arrested after authorities found he matched a description put out by Escambia authorities .

Police believe six to eight people were involved in the homicides , which occurred in the couple 's home in Beulah , west of Pensacola , near the Alabama state line . Watch surveillance video of the home invasion ''

The crime was `` a very well-planned and methodical operation , '' Morgan said .

Wayne Coldiron , 41 ; Leonard Patrick Gonzalez Jr. , 35 , and Leonard Patrick Gonzalez Sr. , 56 , were arrested over the weekend . Coldiron and the younger Gonzalez face charges of murder , robbery and residential home invasion ; the elder Gonzalez faces charges of evidence tampering for allegedly trying to disguise a vehicle spotted at the home .

Morgan said at least three others are persons of interests in the investigation .

`` We expect more arrests to be imminent , '' Morgan told reporters .

Both of the Billingses were shot multiple times , Morgan said , but he would not release further details on their deaths .

Authorities released two surveillance tapes taken from the front and rear of the Billingses ' home . Each shows a vehicle pulling up to the property , and five people dressed in black and wearing masks entering the home through two entrances -- including through a utility door left unlocked , something Morgan said is not uncommon in the community . Authorities believe drivers remained in both of the cars .

Investigators believe one motive in the deaths was robbery , but `` we believe there are other motives , '' Morgan said . He would not say what , if anything , was taken from the home .

Melanie Billings ' biological daughter , Ashley Markham , told reporters the couple initially had 17 children -- two biological children each for Byrd and Melanie Billings , with the rest adopted . Three have died over the years , she said . The couple had no biological children together . Watch Ashley Markham say , `` Love was never scarce '' in Billings home ''

Morgan , however , said the couple had a total of 16 children , with two that have died and others that have grown older and no longer live in the Billingses ' home .

Nine of the couple 's children were home at the time of the incident , Morgan said , and police believe three of them saw the intruders . One managed to flee the home and seek help at a neighbor 's house , the sheriff said .

Coldiron and the younger Gonzalez were being held on $ 1 million bond , according to records posted on the sheriff 's Web site . The senior Gonzalez was being held on $ 250,000 bond .

One of the first three arrested is believed to be the mastermind behind the crime , Morgan said , but would not say which one . Watch a report on the arrests in the complex case ''

Police also released a surveillance photo taken at a Wal-Mart in nearby Gulf Breeze , Florida , recently . Two of the people in the photo are Sumner and the younger Gonzalez , but authorities want to know who the other man is , Morgan said .

The sheriff called the surveillance tapes `` chilling . '' He noted the vehicles were at the home less than 10 minutes , and the five people were in the house less than four minutes .

`` It leads me to believe that this was a very well-planned and methodical operation , '' Morgan said . Watch experts describe the `` military-style '' attack ''

Although the Billingses were well known in the community , the sheriff said authorities are still trying to unravel why they were targeted . He compared their deaths to the slaying of the Clutter family of Kansas , inspiration for Truman Capote 's novel `` In Cold Blood , '' noting the Clutter murders were something the community struggled with for years .

`` It will be a very long time , I believe , until we piece together the truth of why this family was selected , '' Morgan said .

Police believe the suspects might have bought the clothes they wore to the home and were reviewing surveillance tapes and photos from several stores , he said .

Morgan said the crime 's complexity is frustrating for investigators , comparing it to a complicated mathematics or word problem that lacks complete information .

`` It seems as though each phase we complete , while we answer a set of questions , it opens up an additional set of questions . ''

Earlier , he said the complete story , when revealed , is `` going to be a humdinger . ''

Asked whether the suspects entered the home planning to kill the couple , Morgan said authorities do not know .

Markham said earlier the family does not know any of the three suspects . She said the children `` are coping very well '' and are being cared for .

`` They have n't asked too many questions , '' she said , noting that several have disabilities . While the investigation continues , the family is keeping the children 's whereabouts a secret .

CNN 's David Mattingly contributed to this report .

@highlight

Gary Lamont Sumner faces murder charge in deaths of Byrd , Melanie Billings

@highlight

More arrests imminent , Escambia County county sheriff says

@highlight

Surveillance video shows 5 masked people storming home in Beulah , Florida

@highlight

Robbery a motive , but `` we believe there are other motives , '' sheriff says